Self-propelled pneumatic tools for making small diameter holes through soil are well known. Such tools are used to form holes for pipes or cables beneath roadways without need for digging a trench across the roadway. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose which enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth. The striker impacts against the front wall (anvil) of the interior of the tool body, causing the tool to move violently forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker moves back for another blow, resulting in incremental forward movement through the soil. Exhaust passages are provided in the tail assembly of the tool to allow spent compressed air to escape into the atmosphere.
Most impact ground piercing tools of this type have a valveless air distributing mechanism which utilizes a stepped air inlet. The step of the air inlet is in sliding, sealing contact with a tubular cavity in the rear of the striker. The striker has radial passages through the tubular wall surrounding this cavity, and an outer bearing surface of enlarged diameter at the rear end of the striker. This bearing surface engages the inner surface of the tool body.
Air fed into the tool enters the cavity in the striker through the air inlet, creating a constant pressure which urges the striker forward. When the striker has moved forward sufficiently far so that the radial passages clear the front end of the step, compressed air enters the space between the striker and the body ahead of the bearing surface at the rear of the striker. Since the cross-sectional area of the front of the striker is greater than the cross-sectional area of its rear cavity, the net force exerted by the compressed air now urges the striker backwards instead of forwards. This generally happens just after the striker has imparted a blow to the anvil at the front of the tool.
As the striker moves rearwardly, the radial holes pass back over the step and isolate the front chamber of the tool from the compressed air supply. The momentum of the striker carries it rearward until the radial holes clear the rear end of the step. At this time the pressure in the front chamber is relieved because the air therein rushes out through the radial holes and passes through exhaust passages at the rear of the tool into the atmosphere. The pressure in the rear cavity of the striker, which defines a constant pressure chamber together with the stepped air inlet, then causes the striker to move forwardly again, and the cycle is repeated.
In some prior tools, the air inlet includes a separate air inlet pipe, which is secured to the body by a radial flange having exhaust holes therethrough, and a stepped bushing connected to the air inlet pipe by a flexible hose. These tools have been made reversible by providing a threaded connection between the air inlet sleeve and the surrounding structure which holds the air inlet concentric with the tool body. The threaded connection allows the operator to rotate the air supply hose and thereby displace the stepped air inlet rearwardly relative to the striker. Since the stroke of the striker is determined by the position of the step, i.e., the positions at which the radial holes are uncovered, rearward displacement of the stepped air inlet causes the striker to hit against the tail nut at the rear of the tool instead of the front anvil, driving the tool rearward out of the hole. See, for example, Wentworth et al. U.S. Pat. Nos. 5,025,868 and 5,337,837.
Expanders are tapered, ring-shaped shells that fit over the tapered nose portion of an earth boring tool in order to widen the hole made by the tool as it passes through the ground. In this manner, a 4-inch diameter tool may be used to make a 6- or 8-inch diameter hole. The tool is often sent through to make an initial bore, and then sent through a second time with the expander in order to widen the existing hole and/or crack an existing pipe. According to a known method, a plastic pipe may be attached to the back of the expander with the above described reversible tool inside the pipe so that the pipe is installed as the tool bores through the soil, with or without additional widening of the bore. The tool body is disposed inside the replacement pipe, and in this arrangement friction between the expander and the soil serves to keep the tool and expander from moving backward during the rearward stroke of the striker.
Problems are encountered as the tool advances further into the ground and the weight of the pipe being drawn behind the tool grows progressively greater. The ground piercing tool relies on friction with the surrounding soil to prevent it from moving backward during the rearward stroke of the striker the same distance the tool moved forward when the striker made its forward impact. The elasticity of the pipe drawn behind the tool can counteract this frictional force, slowing and eventually stopping the tool.
To remedy this difficulty, it is known to attach a cable to the front end of the impact ground piercing tool and use a winch to apply a continuous pulling force to the tool in the forward direction. The cable is threaded through the existing pipeline or a pilot hole and serves to keep the tool moving and prevent it from deviating from its proper course. While it is possible to push on the existing pipeline from behind instead of pulling on the ground piercing tool using a cable, the pushing operation is less effective because it increases the chance that the ground piercing tool will deviate from the desired path.
In most pipe replacement operations, entry and exit pits must be dug at the end of the run. The cable used to pull on the ground piercing tool emerges into the exit pit and must be directed up to the winch, which is generally a large, truck mounted unit with a capacity greatly exceeding the amount of pulling force needed to keep the ground piercing tool moving. Setting up the winch for pulling is a cumbersome operation because the winch cannot readily be placed and secured directly in the exit pit. The present invention addresses this problem.
Directional boring machines are large, hydraulic rod pushing and pulling systems with the capability of rotating the string of rods (drill string). See, for example, units shown in Malzahn U.S. Pat. Nos. 4,945,999 and 5,070,948, Cherrington U.S. Pat. No. 4,697,775 (RE 33,793), Dunn U.S. Pat. No. 4,953,633 and Deken et al. U.S. Pat. No. 5,242,026. Large directional boring machines generate tremendous reaction forces, and thus it has become standard practice to secure the corners of the machine to the ground with tubular stakes several feet in length. A pneumatic ground piercing tool of the same type used to pull the replacement pipe is fitted on the inside of the tubular stake and used to drive the stake into the ground in a few seconds. Reversing the tool causes the tool to emerge from the ground, leaving the stake intact. The stake has been removed by replacing the tool in the stake, removably securing an insert to the stake in a position behind the tool, and then running the tool in reverse.